Radioactivity & Medicine II

Nuclear Medicine
http://www.colinstudholme.net/research/ipag/mrdspect/mrspect3.html
http://www.mwit.ac.th/~Physicslab/applet_04/atom2/Alphae.gif
Background Image courtesy of Dr. Bill Moore, Dept. of Radiology, Stony Brook Hospital
Nuclear Medicine
– Gamma Ray Imaging
• The ionizing radiation employed in most diagnostic nuclear medical imaging is no
different from that employed in x-ray imaging.
• Both involve the detection of photons emerging from the patient’s body however it
depends on where the source is located with respect to the patient.
• X-rays are high energy photons that originate in an extranuclear source.
• However, the gamma rays used in nuclear medicine are intranuclear or produced by the
decay of unstable atomic nuclei.
• Ernest Rutherford (1897) discovered that the emissions of certain radioactive elements
could be detected using a zinc sulfide screen, producing tiny flashes of light called
scintillations.
• Applied medically it become possible to use that isotopes could be introduced into a
patient, where the photons emitted could be identified by newer scintillation
detectors, and an image could be produced of the distribution of the isotope within the
body.
Nuclear Medicine
– Gamma Ray Imaging
• The basics of using gamma rays to image is nuclear medical technique called
a gamma camera.
• The gamma camera consists of three basic parts
1. Collimators
2. Scintillation detector (Scintillator or Photomultiplier tube (PMT)).
3. Electronics & computer elements
http://www.paec.gov.pk/kiran/images/800by600/dual-head-gamma-camera.jpg
The Gamma Camera
• The gamma camera was invented by H. Anger in the1960s and is often referred to as the
Anger camera
• An Anger camera consists of a collimator, placed between the detector surface and the
patient, and the collimators are made out of a highly absorbing material such as lead.
This suppresses gamma rays that deviate substantially from the vertical and acts as a
kind of "lens". The simplest collimators contain parallel holes.
• Depending on the position of the radiation event, the appropriate phototubes are activated.
The positional information is recorded onto film as an analogue image or onto a
computer as a digital image.
• This set-up yields relatively accurate positional information. The intrinsic resolution of two
radiation sources placed immediately on the crystal surface without the collimator is in
the order of 1 mm.
The Gamma Camera
-Collimators
• In an ordinary photographic camera a lens diverts
light rays by refraction to form an image on
the film or detector.
• For gamma rays, the image is formed by a component
called a collimator
• The collimators are usually made out of a thick
sheet of a heavy material usually lead, that is
perforated like a honeycomb by long thin
channels.
Wolbarst, Physics of Radiology, Ch. 42
• The collimator forms an image by selecting only
the rays traveling in (or nearly in) a specific
direction, in which the channels are oriented.
• Gamma rays traveling in other directions are
either blocked by the channel walls or miss the
collimator entirely.
Wolbarst, Physics of Radiology, Ch. 42
The Gamma Camera
-Collimators
•Collimators are “rated” with respect to their
photon energy and resolution/sensitivity.
Low-energy, or “technetium,” collimators,
are “all purpose” collimators (LEAP) or “low
energy high resolution” (LEHR), image
gamma rays less than 200 keV in energy.
99*Tc , 201Tl , 123I and 57Co
Medium-energy, or “gallium,” collimators
for gamma rays 200–300 keV in energy.
67Ga and 111In
High-energy, or “iodine,” collimators for
gamma rays greater than 300 keV in
energy.131I
Wolbarst, Physics of Radiology, Ch. 42
The Gamma Camera
-Collimators
• The collimator preferentially selects the
direction of the incoming radiation.
• Gamma rays traveling at an oblique angle to
the axes of the holes will strike the lead
walls (septa) and not reach the crystal to
be detected.
• This allows only radiation traveling
perpendicular to the crystal surface to
pass and contribute to the resulting
image.
http://tech.snmjournals.org/content/39/2/100/F1.large.jpg
• A certain fraction (about 5%) of photons
striking the septa will pass through them
and reach the crystal; this phenomenon,
which degrades image quality, is known
as septal penetration.
The Gamma Camera
-Collimators
• From low- to medium- to high-energy collimation, the collimators are made longer and
the septa thicker (maintaining septal penetration at or below an acceptably low level,
(5%).
• This, in turn, reduces the overall fraction of emitted rays reaching the crystal.
• To compensate for the resulting lower sensitivity,
the apertures are typically made wider in
progressing from low-to-medium-to-high
energy collimators.
This, however, degrades spatial resolution.
• Therefore, gamma camera images are progressively
poorer in quality for radionuclides whose
gamma rays are emitting low-to-medium-to-high
energy because of a combination of :
Wolbarst, Physics of Radiology, Ch. 13
1. increased septal penetration with increasing photon energy,
2. lower sensitivity because of the longer collimation, and
3. coarser resolution because of the wider apertures.
The Gamma Camera
-Collimators
The Gamma Camera
-Detectors
• The detectors are generally made of a reflective material so that light emitted toward
the sides and front of the crystal are reflected back toward a photomultiplier tube
and get counted.
• This maximizes the amount of light collected and therefore the overall sensitivity of the
detector.
• This also ensures that the amount of light detected is proportional to the energy of the
absorbed gamma ray photon.
Fiber optic light pipe:
• Interposed between the back of the crystal and the entrance window of the PMT (thin
layer of transparent optical gel).
• It optically couples the crystal to the PMT and thus maximizes the transmission (>90%)
of the light signal from the crystal into the PMT.
The Gamma Camera
-Detectors
Crystals vary in thickness from 1/4” - 1”
• ¼” provides the best spatial resolution but lowest sensitivity
• 1” provides the highest sensitivity but coarsest resolution
mostly used for imaging the photons of 18F
3/8”provides the optimum balance between sensitivity and
resolution and is the most widely used for general
gamma camera imaging. ~95% of the photons from
99Tc are absorbed in a 3/8” crystal.
• The resulting light signal is spread out among the PMTs
in a two-dimensional array on the back of the crystal.
• These photons are detected by (PMTs) and based on the
photoelectric effect, from a single photoelectron, a PMT
can produce a cascade of electrons, which yields a
measurable electrical current.
The Gamma Camera
-Thyroid Imaging
• The thyroid is a gland that makes and stores
essential hormones that help regulate the
heart rate, blood pressure, body
temperature, and the rate of chemical
reactions (metabolism) in the body.
• It is located in the anterior neck.
http://www.medicinenet.com/thyroid_scan/article.htm
• The thyroid gland is the, is the main part of
the body that takes up iodine. In a thyroid
scan, iodine is labeled with a radioactive
tracer, and a special camera is used to
measure how much tracer is absorbed
from the bloodstream by the thyroid gland.
If a patient is allergic to iodine,
technetium
can be used as an alternative.
https://s3.amazonaws.com/healthtap-public/htstaging/user_answer/reference_image/8948/large/Thyroid_sca
n.jpeg?1349479012
The Gamma Camera
-Osteosarcoma
• Osteosarcoma is the most common cancerous
bone tumor in kids. The average age at
diagnosis is 15 and both sexes are just as
likely to get this tumor until the late teen
years, when it is more often seen in boys.
• Osteosarcoma is also more commonly seen in people
over age 60.
• Osteosarcoma tends to occur in the bones of the:
Shin (near the knee)
Thigh (near the knee)
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002616/figure/A001650.B9714/?report=objectonly
Upper arm (near the shoulder)
• This cancer occurs most commonly in larger bones and in the area of bone with the fastest
growth rate and Osteosarcoma can occur in any bone.
The Gamma Camera
-Osteosarcoma
How is an Osteosarcoma detected:
• X-ray – used to confirm the presence of a
tumor in the bone
• Magnetic Resonance Imaging (MRI) or
Computed Tomography (CT) scans are
used to determine the extent of the tumor
• Radionuclide Bone scan – uses primarily a
Gamma Camera to confirm primary sites
and identify any additional sites of bone involvement
Chew & Hudson Radionuclide Bone Scanning of Osteosarcoma, AJR 139:49-54, July 1982
• Positron Emission Tomography (PET), gamma camera, or a
Single Photon Emission CT (SPECT) scan -- to find
small tumors or check if treatment is working effectively
• Biopsy -- to remove tissue from the tumor for microscopic
examination by an expert pathologist
Image of the right femur using an x-ray and part of
a whole body gamma camera imaging of a
pyrophospate radiolabeled using 99mTc.
The left is an image of a suspected osteosarcoma
The right is a the gamma camera image showing
uptake in both knees as well as the lower part of
the femur.
The Gamma Camera
-Osteosarcoma
• The bone scan is a well established procedure
in Nuclear Medicine.
• Areas of bone injury or bone destruction are
usually associated with ongoing bone
repair and consequent increased metabolic
activity and calcium turnover.
• The radionuclides which mimic the metabolic
behavior of calcium will localize in this
region of bone repair in increased
concentration relative to normal bone. In
the past 85Sr and 18F were the primary
radionuclides used.
• However, various phosphate compounds
labeled with 99mTc are mostly the
radionuclides of choice
http://jco.ascopubs.org/content/20/1/189.full.pdf+html
• Technetium is metastable and
decays by emitting a 140
keV gamma ray with a
physical half-life of 6 hours
and a biologic half life of 1
day.
99mTc
 99Tc + g
The Gamma Camera
-Osteosarcoma
•However not all bone scans are done with 99mTc.
• Here is a gamma camera image of a bone scan
that was done with 153Sm (Samarium.)
• Samarium is a bone-seeking
radiopharmaceutical that provides both
diagnostic (gamma ray emitter) and
therapeutic (beta emitter) irradiation to
osteoblastic bone metastases.
• The injected radionuclide is 153Sm-EDTMP
(153Sm ethylene diamine tetramethylene
phosphonate), has a high bone uptake due to
the phosphorus concentration in the bones.
http://jco.ascopubs.org/content/20/1/189.full.pdf+html
•Variable uptake of 153Sm-EDTMP in metastatic
osteosarcoma
• Anterior and posterior gamma camera imaging 24
hours after administration of 3 mCi/kg. Note the
more avid radioisotope uptake of the left chest
mass adherent to the pericardium compared
with the spine and right renal metastases.
The Gamma Camera
-Osteosarcoma
7 Cervical
Humerus
12 Thoracic
Spine
Hip bones
5 Lumbar
5 Sacrum
Sacrum
4 Coccyx
Pubic symphysis
http://nasachiropractic.com/clients/4013/images/S
I_joint.jpg
Femur
Patella
Tibia
http://jco.ascopubs.org/content/20/1/189.full.pdf+html
Sacroiliac joint
Whole-body anterior gamma camera imaging of osteosarcoma of the left humerus and additional bone
metastases (left rib, right sacroiliac joint, L5 spine) at 2, 24, 48, and 120 hours (right to left) after 6 mCi/kg of
153Sm-EDTMP.
Summary:
• The radioactive decay of unstable elements allows for medical imaging and
detection of metabolically active sites in the body.
• Radiolabeled drugs are injected into the body and travel to glucose active sites and
subsequent PET or gamma camera scans are performed to locate the activity.
• PET scans are a non-invasive imaging technique and are fused with CT (or MRI)
scans to given anatomical information.
• PET scans make use out of coincident coupled gamma rays from the annihilation
of positron-electron pairs.
• Gamma camera scans provide effective imaging of uptake in bones.